1. Field of the Invention
The present invention generally relates to a welding flux and, more particularly, to a welding flux used for austenitic stainless steel.
2. Description of the Related Art
Stainless steel consists essentially of iron (Fe). Specifically, stainless steel is high-alloy steel containing basic elements such as carbon (C), silicon (Si), manganese (Mn), phosphor (P) and sulfur (S), and further containing alloying elements such as chromium (Cr), nickel (Ni), molybdenum (Mo), nitrogen (N), titanium (Ti) and niobium (Nb). Accordingly, the content of Cr in stainless steel must reach a weight percentage of at least 12 wt. % for forming a compact, continuous passive layer of chromium(III) oxide (Cr2O3). Since Cr atom is more active than Fe atom and reacts easily with oxygen (O) atom in the atmosphere or environment to form a thin layer of Cr2O3 on the surface of the stainless steel, the corrosion factors in the atmosphere or environment are thus prevented from diffusing into the interior of the stainless steel. Furthermore, since stainless steel is provided with excellent resistances of corrosion and abrasion, mechanical strength, formability and weldability, it can be widely utilized in livelihood industry, food industry, pharmaceutical industry, petrochemical industry, nuclear industry, aerospace industry, paper industry, shipbuilding industry and medical device industry. Generally, stainless steel can be classified based on two criteria. Firstly, it can be sorted according to the alloy elements contained therein into two major groups, which are Fe—Cr stainless steel and Fe—Cr—Ni stainless steel. Secondly, it can be sorted according to the microstructure under room temperature into five major groups, which are ferritic, austenitic, martensitic, duplex and precipitation hardening stainless steel. Specifically, austenitic stainless steel belongs to Fe—Cr—Ni stainless steel, and is non-magnetic. The microstructure of austenitic stainless steel is not affected by heating temperature; hence, its mechanical strength cannot be enhanced by heat treatment. Austenitic stainless steel possesses not only excellent formability, weldability and general corrosion resistance, but also excellent pitting resistance. Austenitic stainless steel is widely used in tableware, medical device and kitchenware. Compared to martensitic stainless steel, austenitic stainless steel has a good corrosion resistance; however its abrasion resistance is not as well as martensitic stainless steel.
Tungsten inner gas (TIG) welding is a high-quality arc welding technique. The arc welding of austenitic stainless steel in practice is mainly performed using TIG welding. The TIG welding is performed under a protective atmosphere of an inner gas (e.g. argon gas or helium gas), with an electric arc generated by a tungsten electrode as a welding heat source for melting the joint of two workpieces. The melted parts of the workpieces are rapidly cooled down and then solidified, thus joining the two workpieces together. However, since the power density of TIG welding heat source is not high enough, the joint penetration achievable in single-pass operation without edge preparation is less than 3 mm, and thus the TIG welding is not cost-efficient to join thick section workpieces.
In the case that welding of thick section workpieces (with a thickness more than 3 mm) is required, a conventional welding flux can be applied to the joint of two workpieces, such that the conventional welding flux will be melted to form a molten pool at the joint of the two workpieces. The molten pool is then cooled down to room temperature, resulting in a weld which tightly joins two workpieces. A conventional welding flux includes titanium dioxide (TiO2), silicon dioxide (SiO2), Cr2O3, nickel oxide (NiO) and copper oxide (CuO), which is able to form a deep, narrow weld and suitable for welding of thick workpieces. However, the conventional welding flux is provided with weak surface hardness, which is insufficient for a situation where high abrasion resistance is required.